In the related art, as one of systems which purify exhaust gas of an automobile, there has been known a selective catalytic reduction (SCR) system that reduces NOx to nitrogen and water using ammonia, and zeolite having a copper-carried CHA structure is paid attention as zeolite achieving SCR catalysis.
In this SCR system, a honeycomb unit, in which a large number of through holes allowing the exhaust gas to pass therethrough are arranged in parallel to extend in a longitudinal direction, is used as an SCR catalyst carrier. For example, as one that aims to enhance heat resistance and endurance in a case of being used as an SCR catalyst carrier, zeolite having a CHA structure is disclosed in Patent Literature 1 in which a composition ratio of SiO2/Al2O3 is less than 15 and a particle size is 1.0 to 8.0 μm.
On the other hand, zeolite having a CHA structure is disclosed in Patent Literature 2 in which a composition ratio of SiO2/Al2O3 is less than 15 and the content of alkali is less than 3% by mass. In this case, however, the surface of a honeycomb unit substrate made of another substance needs to be coated with the CHA-structured zeolite to obtain an effect as an SCR catalyst.
A technique is disclosed in Patent Literature 3 in which zeolite is extrusion-molded together with an inorganic binder and the like into a honeycomb shape in producing of a honeycomb unit to be used as an SCR catalyst.
A catalyst composition is disclosed in Patent Literature 4 in which a molar ratio (SAR) of silica to alumina is about 10 to about 30, the content of an exchange nonaluminum transition metal (for example, copper (Cu)) is about 1.5 to about 5 wt % with respect to the total weight of zeolite, and the content of cerium (Ce) is about 1.35 wt % or more.